The temperate bacteriophage P1 has long been a valuable tool for Escherichia coli geneticists. It mediates generalized transduction and has derivatives mediating specialized transduction (Plstd). It has been established that the P1 prophage is a plasmid and that the bacteriophage DNAs are permuted. However, the lytic cycle chromosomal forms and the pathways of virion chromosomal maturation remain to be characterized, for both P1 and mutant derivatives. This problem is to be attacked through lytic cycle chromosomal form isolation, electronmicroscopy, and cleavage analyses. Information thus gained will be combined with a refined analysis of hybrid joints of Plstd, to further elucidate the pathways of Plstd formation.